|Publication number||US4867564 A|
|Application number||US 07/064,371|
|Publication date||Sep 19, 1989|
|Filing date||Jun 22, 1987|
|Priority date||Jun 22, 1987|
|Publication number||064371, 07064371, US 4867564 A, US 4867564A, US-A-4867564, US4867564 A, US4867564A|
|Inventors||Harold E. Sweeney, Donald A. Leonard|
|Original Assignee||Gte Government Systems Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (3), Non-Patent Citations (2), Referenced by (11), Classifications (11), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
Ser. No. 064,375 filed June 22, 1987, "METHOD OF REMOTELY DETECTING SUBMARINES USING A LASER."
1 Field of the Invention
The present invention relates to the measurement of sub-surface ocean temperature profiles and more particularly to an improved method of and apparatus for remotely measuring such temperature profiles from an aircraft or the like.
2. Description of the Prior Art
Profiles of ocean water temperature as a function of depth are important basic scientific data used by the oceanographic community for several purposes including determination of the effect on climate. A technique employed in the past for measuring such profiles involves the use of many temperature sensing elements spaced on a cable and towed by a ship. This is costly, time consuming and generally unsuited to high spatial and temporal resolution coverage of large ocean areas. Furthermore, the technique cannot be used for mapping ocean temperature profiles, that is, temperatures over wide areas of water, because the measuring time is too long compared to the time over which sea temperatures vary.
Another technique is described in Patent No. 4,123,160 in which a laser beam is used to illuminate the water and observation is made of the Raman scatter from the monomer and hydrogen bonded polymeric forms of water, the ratio of which is a function of temperature. This technique is vulnerable to interference from high background illumination, such as sunlight, because of the relatively wide optical bandwidth of the Raman scattering. Furthermore, in this technique there is differential absorption over the Raman band as light transits the water column. Depolarization effects of the water column also limit the effectiveness of the technique when polarization spectroscopy is employed.
Still another laser remote sensing method has been used in limited experiments, see "SPEED OF SOUND AND TEMPERATURE IN THE OCEAN BY BRILLOUIN SCATTERING" by Hirschberg, et al., Applied Optics, Aug. 1984, pages 2624-2628, inclusive. This method relies on the wavelength shift associated with Brillouin scattering from the water. This shift, however, is small so that extremely high resolution is required in optical measurement of the wavelength shift. Typically a Fabry-Perot interferometer is used to resolve the Brillouin shift. However, an interferometer requires a well collimated light source which generally is incompatible with remote sensing applications, that is, because of spreading, light must be collected from a much larger field of view than is possible with an interferometer.
This invention is directed to the measurement of sub-surface ocean temperatures while avoiding these disadvantages.
A general object of the invention is the provision of a technique of remotely and rapidly measuring ocean temperature profiles without interference from high background illumination, such as sunlight.
Another object is the provision of such measuring apparatus having relatively high overall efficiency.
Still another object is the provision of a method of and apparatus for remotely measuring ocean temperature profiles without the need for a precision interferometer.
A further object is the provision of such a technique and apparatus having relatively high sensitivity.
These and other objects of the invention are achieved by using the "self-heterodyne" of the wavelength-shifted Brillouin scatter with the unshifted Rayleigh scatter mainly from impurities in the water. The "self-heterodyne" action allows mixing of these signals from each volume element of the illuminated water column independently and measuring the frequency of the heterodyne signal. Since this frequency is directly related to the water temperature and since time is directly proportional to depth, the resultant time-temperature pattern is equivalent to a temperature-depth profile.
For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawings wherein:
FIG. 1 is a schematic view of an aircraft over a body of water and involved in the practice of the invention.
FIG. 2 is a schematic block diagram of the transmitter and receiver components comprising the apparatus embodying the invention.
FIG. 3 is a diagram illustrating frequency products resulting from mixing Rayleigh and Brillouin backscatter components in accordance with the practice of the invention.
FIG. 4 is a set of curves depicting the relation of ocean temperature resolution and depth for different monitoring altitudes.
It should be noted that the same reference numerals identify identical components in each of the figures.
For a better understanding of the subject invention, reference is made to the following description and appended claims in conjunction with the above-described drawings.
Referring now to the drawings, FIG. 1 depicts an aircraft 10 in flight at apredetermined altitude over a body 12 of water such as an ocean having a transmitter 13 for directing a laser beam 14 into the water and a receiver15 for receiving a return signal 16 from the water. Transmitter 13 generates a pulsed output beam 14 having a wavelength selected to penetrate the water 12, and return signal 16 contains Brillouin and Rayleigh backscatter components. In accordance with this invention, receiver 15 processes the Brillouin and Rayleigh signals to produce data indicative of the temperature at various depths to provide temperature-depth information.
A more detailed block diagram of transmitter 13 and receiver 15 along with associated optics is shown in FIG. 2. Transmitter 13 comprises a laser 18 controlled by a timer 19 to produce output laser pulses 20. The output of laser 18 preferably is in the blue-green part of the spectrum (4500 to 5500 Å in wavelength) and has a typical pulse repetition frequency of 100 Hz. A laser useful for this purpose is a frequency doubled Nd:YAG at awavelength of 5300 Å. Laser output 20 is directed by mirrors 21, 22 and23 to the target area, in this application, the sea water 12 below aircraft10.
Return signal 16 contains Brillouin and Rayleigh backscatter components resulting from the interaction of the laser pulses with the sea water. Mirrors 23, 24 and 25 direct the return light 16 to the receiver 15 comprising photodetector 26 having a cathode 26a, discriminator 27, analog-to-digital converter 28, analyzer unit 29 and a recorder 30. Phototector 26 preferably is a photomultiplier tube having a photocathode on which the return light 16 is incident. The PM tube converts the opticalenergy (photons) to electrons (current) while simultaneously amplifying thesignal. The photocathode also performs the additional important function ofmixing the Brillouin and Rayleigh components of signal 16 to provide a difference frequency, called the optical heterodyne frequency. The Brillouin backscatter component is frequency-shifted by the acoustic properties of the water whereas the Rayleigh backscatter component has a constant frequency and is analogous to the "local oscillator" in a superheterodyne receiver. The acoustic velocity of the water is a functionof water temperature.
Referring now to FIG. 3, the Brillouin and Rayleigh components incident on cathode 26a of photodetector 26 are represented by curves 37; and 33 at frequencies f1 and f3, respectively, and by curve 34 at frequency f2. The signals at frequencies f1 and f3 are the frequency shifted Brillouin components, and the signal at f2 is the frequency unshifted Rayleigh component, the latter functioning as a "localoscillator" in an analogy to a superheterodyne receiver. The two differencefrequencies between f3 and f2 and between f2 and f1 arethe same frequency, fB and are additive in the baseband so that the process results in increased sensitivity. Moreover, the phase relationshipof the Brillouin and Rayleigh components at photodetector 26 is correct forefficient heterodyning since both components are received from the same angle of view however wide and are produced by the same laser pulse stream.
The output (current) of photodetector 26 passes to discriminator 27, such as a delay line discriminator, which produces a voltage proportional to the baseband frequency fB. Converter 28 changes the output of discriminator 27 into digital form for analysis in analyzer 29. Analyzer 29 receives the digital frequency fB and, using the equations below, translates it into a signal or number representative of the water temperatures. By way of example, analyzer 29 may comprise a computer whichcompares the digital frequency input with a reference table to derive the equivalent temperature. The output of analyzer 29 is stored in recorder 30.
The accuracy of temperature measurement by analysis of Brillouin backscatter may be shown mathematically. The relation of signal-to-noise ratio (SNR) to the standard deviation of frequency measurement accuracy for a radar is given by the following expression. ##EQU1##
This standard deviation of the frequency also depends on a parameter To which is usually taken to be the laser pulse width. In this case, however, the self broadening of the Brillouin shifted lines produces a Brillouin bandwidth of 480 MHz (See Hirschberg, J. G., et al, "SPEED OF SOUND AND TEMPERATURE IN THE OCEAN BY BRILLOUIN SCATTERING," Applied Optics, 23, 2624 (1984)). This requires that an effective To of 2.1 ns be used in equation (1) rather than the laser pulse duration.
The Brillouin backscatter frequency fB is a function of the acoustic velocity vs and the laser optical frequency, fo, as follows
fB =2nvs (1/c)fo (2)
where n is the index of refraction and c is the velocity of light in free space. Differentiating equation (2) with respect to temperature yields
dfB /dT=2n(1/c)fo (dvs /dT) (3)
The sound velocity vs is given as a function of temperature by the following expression.
vs =1400+5.02T-0.055T2 +0.003T3 (4)
which, when differentiated with respect to temperature and evaluated at 10° C., gives dvs /dT=4.82 m/s-° C. Substituting this value into equation (3) and using 6.54×1014 Hz as the optical frequency for an assumed transmitter wavelength of 459 nm, the result obtained is
dfB /dT=27.95 MHz/° C. (5)
The temperature accuracy as a function of SNR can be obtained by dividing equation (1) by dfb /dT. The resultant temperature accuracy expressedas a function of depth for a representative system is shown in FIG. 4.
While the invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teaching of the invention without departing from its essential teachings.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US3474253 *||Jun 2, 1966||Oct 21, 1969||Us Navy||Method and apparatus for optically detecting acoustic disturbances|
|US4123160 *||Dec 20, 1976||Oct 31, 1978||Bernard Caputo||Method and apparatus for remotely measuring sub-surface water temperatures|
|US4411525 *||Aug 11, 1981||Oct 25, 1983||Fuji Photo Optical Co., Ltd.||Method of analyzing an object by use of scattering light|
|1||*||Speed of Sound & Temperature in the Ocean by Brillouin Scattering, by Joseph G. Hirschberg, et al., Applied Optics/vol. 23, No. 15, 8/1/84 pp. 2624 2628, inclusive.|
|2||Speed of Sound & Temperature in the Ocean by Brillouin Scattering, by Joseph G. Hirschberg, et al., Applied Optics/vol. 23, No. 15, 8/1/84 pp. 2624-2628, inclusive.|
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US4948958 *||Aug 1, 1989||Aug 14, 1990||Gte Government Systems Corporation||Remote subsurface water temperature measuring apparatus with brillouin scattering|
|US4962319 *||Aug 29, 1989||Oct 9, 1990||Gte Government Systems Corporation||Remote subsurface water temperature measuring apparatus with Brillouin scattering|
|US4973853 *||Jul 28, 1989||Nov 27, 1990||Gte Government Systems Corporation||Remote subsurface water temperature measuring apparatus with Brillouin scattering|
|US5026999 *||Apr 9, 1990||Jun 25, 1991||Gte Government Systems Corporation||Method of remotely measuring subsurface water temperatures by stimulated raman scattering using stimulated brillouin backscattering|
|US5110217 *||Oct 31, 1990||May 5, 1992||Gte Government Systems Corporation||Method for optically and remotely sensing subsurface water temperature|
|US5262644 *||Dec 16, 1992||Nov 16, 1993||Southwest Research Institute||Remote spectroscopy for raman and brillouin scattering|
|US6786633 *||Feb 1, 2002||Sep 7, 2004||Maquet Critical Care Ab||Method and arrangement for acoustically determining a fluid temperature|
|US7283426 *||Mar 30, 2004||Oct 16, 2007||Bae Systems Information And Electronic Systems Integration Inc.||Method and apparatus for detecting submarines|
|US8152366 *||Jan 22, 2009||Apr 10, 2012||University Of Delaware||Estimation of subsurface thermal structure using sea surface height and sea surface temperature|
|US20060083111 *||Mar 30, 2004||Apr 20, 2006||Grasso Robert J||Method and apparatus for detecting submarines|
|US20090187369 *||Jan 22, 2009||Jul 23, 2009||University Of Delaware||Estimation of subsurface thermal structure using sea surface height and sea surface temperature|
|U.S. Classification||356/484, 374/117, 374/E11.018, 374/E13.011, 356/43|
|International Classification||G01K13/10, G01K11/12|
|Cooperative Classification||G01K11/12, G01K13/10|
|European Classification||G01K11/12, G01K13/10|
|Jun 22, 1987||AS||Assignment|
Owner name: GTE GOVERNMENT SYSTEMS CORPORATION, CONNECTICUT
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:SWEENEY, HAROLD E.;LEONARD, DONALD A.;REEL/FRAME:004733/0389
Effective date: 19870619
|Dec 14, 1992||FPAY||Fee payment|
Year of fee payment: 4
|Apr 29, 1997||REMI||Maintenance fee reminder mailed|
|Sep 21, 1997||LAPS||Lapse for failure to pay maintenance fees|
|Dec 2, 1997||FP||Expired due to failure to pay maintenance fee|
Effective date: 19970924